Preparation method and application of solid spherical sodium ferrous sulfate composite positive electrode material

Abstract

The present invention discloses a preparation method and application of a solid spherical sodium ferrous sulfate composite positive electrode material. The preparation method comprises the following steps: step 1: adding a conductive carbon material to deionized water, and ultrasonically obtaining a conductive carbon solution; adding ascorbic acid, anhydrous sodium sulfate, and ferrous sulfate heptahydrate prepared based on hematite sediment to deionized water, respectively, and stirring to obtain a light blue solution; pouring the conductive carbon solution into the light blue solution in a mass ratio of 1:1 and stirring to obtain a precursor solution; step 2: spray-drying the precursor solution to obtain an intermediate phase; step 3: grinding the intermediate phase uniformly and placing it in a tube furnace, and calcining it under an inert atmosphere to obtain a solid spherical sodium ferrous sulfate composite positive electrode material. The sodium ferrous sulfate composite positive electrode material prepared by the preparation method of the present invention has a uniform and continuous conductive network, enhances the intrinsic electronic conductivity of sodium ferrous sulfate and the sodium ion transmission rate, and improves the cyclic stability of the material and realizes waste recycling.

Description

Technical Field

[0001] The present invention belongs to the field of sodium ion battery positive electrode materials, and in particular relates to a preparation method and application of a solid spherical sodium ferrous sulfate composite positive electrode material. Background Art

[0002] As the demand for energy storage grows, lithium-ion batteries are increasingly used in society. However, lithium resources are scarce and unevenly distributed on Earth, and the widespread use of lithium-ion batteries is driving up their costs. Compared to lithium, sodium is abundant. The lithium content in the Earth's crust is only 0.0065%, while the sodium content is 2.75%, over 400 times that of the former. Sodium-ion batteries share similar electrochemical reaction mechanisms with lithium-ion batteries, making them an ideal complement to lithium-ion batteries. The positive electrode material is a key component of sodium-ion batteries, significantly impacting their energy and power density. The development of positive electrode materials determines the application of sodium-ion batteries.

[0003] Among the numerous cathode materials for sodium-ion batteries, polyanionic compounds are considered the most promising due to their excellent structural stability, safety, and high voltage platform. Sodium ferrous sulfate (SF4) is an ideal cathode material for sodium-ion batteries due to its low cost, non-toxicity, unique three-dimensional sodium ion diffusion channels, and high voltage platform. However, its large band gap poses challenges to the transition of electrons from the valence band to the conduction band, resulting in a decrease in intrinsic carrier concentration, leading to slow kinetics and poor rate performance.

[0004] Currently reported methods for improving the kinetic and rate performance of sodium ferrous sulfate mainly include effective strategies such as doping, carbon coating, and constructing a well-structured carbon network. Although these methods have opened up broad avenues for improving sodium storage, the improvement of ion / electron transport is still limited. In addition, the complex operation process and the use of toxic reagents have seriously hindered the commercialization process. At present, spray drying technology obtains high-purity and high-activity cathode materials by quickly drying the solution into powder. Therefore, the selection of a simple and environmentally friendly preparation method is crucial to achieve high-performance and scalable sodium ferrous sulfate cathode materials. Considering that a large amount of hematite slag is generated in the metallurgical industry, it can be prepared into high-purity ferrous sulfate heptahydrate by a wet process, and the production of sodium ferrous sulfate cathode materials will consume a large amount of ferrous sulfate heptahydrate. Therefore, the high-value reuse of metallurgical waste slag will form a closed-loop design of waste and new energy materials. Summary of the Invention

[0005] This invention aims to provide a method for preparing and applying a solid spherical sodium ferrous sulfate composite cathode material. Using metallurgical waste as raw material significantly reduces production costs, simplifies the experimental process, and improves production efficiency. The simple carbon composite effectively addresses the low intrinsic electronic conductivity and poor rate performance of sodium ferrous sulfate. The material, when used in sodium-ion batteries, exhibits high capacity and excellent cycle performance.

[0006] The first aspect of the present invention provides a method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material, comprising the following steps:

[0007] Step 1: Add the conductive carbon material to deionized water and sonicate for 6-12 hours to obtain a uniformly dispersed conductive carbon solution; add ascorbic acid, anhydrous sodium sulfate, and ferrous sulfate heptahydrate prepared based on hematite sediment to deionized water respectively, and stir thoroughly on a magnetic stirrer to obtain a light blue solution; then pour the conductive carbon solution into the light blue solution in a mass ratio of 1:1 and stir to obtain a precursor solution;

[0008] Step 2: spray drying the precursor solution to obtain an intermediate phase;

[0009] Step 3: Grind the intermediate phase evenly and place it in a tube furnace, and calcine it under inert atmosphere to obtain a solid spherical sodium ferrous sulfate composite positive electrode material.

[0010] In one embodiment, the rotation speed of the magnetic stirrer is 300-1000 rpm, and the stirring time on the magnetic stirrer is 1-10 h.

[0011] In one embodiment, the ferrous sulfate heptahydrate prepared based on hematite sludge is characterized in that the preparation method is a wet production process, specifically including leaching, reduction and crystallization.

[0012] In one embodiment, the conductive carbon material is any one or more of Ketjen black, carbon nanotubes, graphene, acetylene black, graphite, and Super P (conductive carbon black); the composite mass of the conductive carbon material is 3%-10% of the mass of the sodium ferrous sulfate positive electrode material.

[0013] In one embodiment, the stoichiometric ratio of anhydrous sodium sulfate to ferrous sulfate heptahydrate is 2:(3-4).

[0014] In one embodiment, the mass of the ascorbic acid is 20%-60% of the mass of the ferrous sulfate heptahydrate.

[0015] In one embodiment, the spray drying temperature is set to 180-250° C.; the calcination temperature is 350-400° C.; and the calcination time is 8-18 hours.

[0016] In one embodiment, the inert atmosphere is one of argon, nitrogen, and a mixture of an inert gas and a reducing gas.

[0017] In a second aspect, the present invention also provides a sodium ion battery, comprising a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the positive electrode and the negative electrode are located on both sides of the diaphragm, and the electrolyte is located between the positive electrode and the negative electrode. The positive electrode is a positive electrode sheet prepared by using the solid spherical sodium ferrous sulfate composite positive electrode material prepared by the preparation method of the solid spherical sodium ferrous sulfate composite positive electrode material described above, and the positive electrode sheet is a round sheet with a diameter of 14 mm; the negative electrode is a sodium metal sheet, wherein the diameter of the sodium metal sheet is 16 mm and the thickness is 0.2 to 0.4 mm; the electrolyte is 0.1 mL of a sodium perchlorate / ethylene carbonate / propylene carbonate / fluoroethylene carbonate solution with a concentration of 1 mol / L; and the diaphragm is a glass fiber diaphragm with a diameter of 19 mm.

[0018] Beneficial effects of the present invention:

[0019] (1) The solid spherical sodium ferrous sulfate composite positive electrode material prepared by the present invention has a simple operation method, a high safety factor, is suitable for large-scale production, and the prepared material has a high battery capacity and excellent cycle stability.

[0020] (2) The present invention uses a spray drying method to prepare a solid spherical sodium ferrous sulfate composite cathode material. The resulting intermediate phase is Na2Fe(SO4)2·4H2O, which can be converted into the target sodium ferrous sulfate cathode material by simply controlling the temperature and humidity. The spray drying method used is conducive to the formation of a spherical morphology and reduces the calcination time.

[0021] (3) The present invention proposes the use of conductive carbon materials for in-situ compounding, which self-assembles with sodium ferrous sulfate to form a dense solid spherical structure, effectively improving the cyclic stability of the material.

[0022] (4) The raw materials for preparing the solid spherical sodium ferrous sulfate composite positive electrode material of the present invention are cheap and abundant in source, and are suitable for large-scale industrial production.

[0023] (5) The conductive carbon material provided by the technical solution of the present invention is evenly mixed with the sodium ferrous sulfate positive electrode material, and the rate performance of the material is significantly improved.

[0024] (6) The technical solution of the present invention realizes the organic combination of metallurgical waste slag and new energy, reduces environmental pollution and lowers the production cost of sodium ion battery positive electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 The present invention provides a schematic flow chart of a method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying.

[0027] Figure 2 This is the (XRD pattern) of the sodium ferrous sulfate positive electrode material prepared in Comparative Example 1 without adding conductive carbon material.

[0028] Figure 3 This is a cycle performance diagram of a sodium ion battery prepared using the sodium ferrous sulfate positive electrode material prepared in Comparative Example 1 without adding conductive carbon material at a C / 5 rate.

[0029] Figure 4 This is a scanning electron microscope (SEM) image of the sodium ferrous sulfate positive electrode material prepared in Comparative Example 1 without adding conductive carbon material.

[0030] Figure 5 This is the (XRD) diagram of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 1.

[0031] Figure 6 This is a cycle performance diagram of a sodium ion battery prepared using the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 1 at a C / 5 rate.

[0032] Figure 7 This is a scanning electron microscope (SEM) image of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 1.

[0033] Figure 8 This is the (XRD) diagram of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 2.

[0034] Figure 9 This is a cycle performance diagram of a sodium ion battery prepared using the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 2 at a C / 5 rate.

[0035] Figure 10 This is a scanning electron microscope (SEM) image of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 2.

[0036] Figure 11 This is the XRD pattern of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 3.

[0037] Figure 12 This is a cycle performance diagram of a sodium ion battery prepared using the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 3 at a C / 5 rate.

[0038] Figure 13 This is a scanning electron microscope (SEM) image of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 3.

[0039] Figure 14 This is a scanning electron microscope (SEM) image of a sodium ion battery prepared with the sodium ferrous sulfate positive electrode material prepared in Comparative Example 2 without adding conductive carbon material after cycling at a C / 5 rate.

[0040] Figure 15 This is a transmission electron microscope (TEM) image of the solid spherical sodium ferrous sulfate composite positive electrode material prepared in Example 4. DETAILED DESCRIPTION

[0041] Hereinafter, a method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material of the present invention will be described in detail with reference to exemplary embodiments.

[0042] See also Figure 1 , Figure 1 The figure is a flow chart of the method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided by the present invention. The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying comprises the following steps:

[0043] S1: Add a conductive carbon material to deionized water and sonicate for 6-12 hours to obtain a uniformly dispersed conductive carbon solution; add ascorbic acid, anhydrous sodium sulfate, and ferrous sulfate heptahydrate prepared based on hematite sediment to deionized water, respectively, and stir thoroughly on a magnetic stirrer to obtain a light blue solution; then pour the conductive carbon solution into the light blue solution in a mass ratio of 1:1 and stir to obtain a precursor solution;

[0044] S2: spray drying the precursor solution to obtain an intermediate phase;

[0045] S3: Grind the intermediate phase evenly and place it in a tube furnace, and calcine it under inert atmosphere to obtain a solid spherical sodium ferrous sulfate composite positive electrode material.

[0046] Specifically, the ultrasonic time in step S1 is 6-12 hours, that is, it can be 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or other values ​​within the above range.

[0047] The wet process used to prepare ferrous sulfate heptahydrate from hematite sludge in step S1 specifically includes leaching, reduction and crystallization.

[0048] The rotation speed of the magnetic stirrer in step S1 is 300-1000 rpm, that is, it can be 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, or other values ​​within the above range.

[0049] The stirring time of the magnetic stirrer in step S1 is 1-10 h, that is, it can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, or other values ​​within the above range.

[0050] The mass ratio of the added deionized water to the added conductive carbon material in step S1 is (4-7):1, that is, it can be 4:1, 5:1, 6:1, 7:1, or other ratios within the above range.

[0051] The mass ratio of the added deionized water to the sodium ferrous sulfate precursor in step S1 is (4-7):1, that is, it can be 4:1, 5:1, 6:1, 7:1, or other ratios within the above range.

[0052] The conductive carbon material in step S1 may be any one or more of Ketjen black, carbon nanotubes, graphene, acetylene black, graphite, and SuperP (conductive carbon black);

[0053] The mass of the conductive carbon material composite in step S1 is 3%-10% of the mass of the sodium ferrous sulfate positive electrode material, that is, it can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or other values ​​within the above range.

[0054] The stoichiometric ratio of anhydrous sodium sulfate to ferrous sulfate heptahydrate in step S1 is 2:(3-4), that is, it can be 2:3, 2:3.2, 2:3.4, 2:3.6, 2:3.8, 2:4, or other values ​​within the above range.

[0055] The mass of the ascorbic acid in step S1 is 20%-60% of the mass of the added ferrous sulfate heptahydrate, that is, it can be 20%, 30%, 40%, 50%, 60%, or other values ​​within the above range.

[0056] The spray drying temperature in step S2 is set to 180-250°C, that is, it can be 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, or other values ​​within the above range.

[0057] The calcination temperature in step S3 is 350-400°C, that is, it can be 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, or other values ​​within the above range.

[0058] The calcination time in step S3 is 8-18 hours, that is, it can be 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, or other values ​​within the above range.

[0059] The inert atmosphere in step S2 is one of argon, nitrogen, and a mixture of an inert gas and a reducing gas.

[0060] In this example, a solid spherical sodium ferrous sulfate composite cathode material was prepared using the aforementioned method. During the preparation of the sodium ferrous sulfate solution, the material was in situ composited with a conductive carbon material, self-assembling to form a dense, solid spherical structure, effectively improving the material's cycling stability. The added conductive carbon material formed an efficient conductive network, effectively enhancing the material's electronic conductivity and rate capability.

[0061] Based on the foregoing, this embodiment also provides a sodium ion battery prepared by spray drying a solid spherical sodium ferrous sulfate composite positive electrode material, comprising a positive electrode, a negative electrode, a diaphragm and an electrolyte, wherein the positive electrode and the negative electrode are located on both sides of the diaphragm, and the electrolyte is located between the positive electrode and the negative electrode. It is characterized in that the positive electrode is a positive electrode sheet prepared from the carbon composite ferrous sulfate positive electrode material, and the positive electrode sheet is a round sheet with a diameter of 14 mm; the negative electrode is a sodium metal sheet, wherein the diameter of the sodium metal sheet is 16 mm and the thickness is 0.2 to 0.4 mm; the electrolyte is 0.1 mL of a sodium perchlorate / ethylene carbonate / propylene carbonate / fluoroethylene carbonate solution with a concentration of 1 mol / L; and the diaphragm is a glass fiber diaphragm with a diameter of 19 mm.

[0062] The solid spherical sodium ferrous sulfate composite positive electrode material prepared according to the preparation method of this embodiment is applied to a sodium ion battery, so that the solid spherical sodium ferrous sulfate composite electrode sheet prepared by the above preparation method also has the advantages of a solid spherical sodium ferrous sulfate composite positive electrode material.

[0063] In order to better understand the present invention, the content of the present invention is further explained below with reference to specific examples, but the content of the present invention is not limited to the following examples.

[0064] Comparative Example 1

[0065] The present invention discloses a method for preparing a solid spherical sodium ferrous sulfate positive electrode material by spray drying, comprising the following steps:

[0066] Step 1: Ascorbic acid accounting for 20% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3 are added to deionized water, wherein the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, and the mass ratio of the added deionized water to the sodium ferrous sulfate precursor is 5:1. The mixture is stirred on a magnetic stirrer for 1 hour to obtain a light blue solution, thereby obtaining a precursor solution;

[0067] Step 2: spray drying the precursor solution obtained in step 1 at 200° C. to obtain an intermediate phase;

[0068] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate positive electrode material.

[0069] This comparative example prepares a solid spherical sodium ferrous sulfate positive electrode material that is not composited with a conductive carbon material. The resulting sodium ferrous sulfate positive electrode material is used to prepare a positive electrode sheet. The positive electrode sheet is made by vacuum drying a slurry coated on aluminum foil at 120°C and then cutting it into pieces. The slurry is made by mixing the positive electrode material with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1, and then adding 1-methyl-2-pyrrolidone as a dispersant. The electrolyte is 1M NaClO4 dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC). The outer shell is a stainless steel shell, the negative electrode is a metal sodium sheet, and a glass fiber separator is used to separate the positive and negative electrodes. The battery is assembled into a button cell.

[0070] Figure 2 The sodium ferrous sulfate (Na 2.5 Fe 1.75 (SO4)3) X-ray diffraction (XRD) pattern of the positive electrode material. It can match the standard X-ray diffraction standard card of sodium ferrous sulfate, that is, Na 2.5 Fe 1.75 (SO4)3.

[0071] Figure 3 This is a cycle performance test of a button cell assembled with sodium ferrous sulfate positive electrode material without composite conductive carbon material in the voltage range of 2.0 to 4.5V. The first discharge capacity is about 68mAh / g under C / 5 rate conditions.

[0072] Figure 4 It is sodium ferrous sulfate (Na 2.5 Fe 1.75 (SO4)3) Scanning electron microscope (SEM) image of the cathode material. It can be seen that the spray drying method can produce solid spherical sodium ferrous sulfate cathode material with a rough surface. The spherical structure is formed by self-assembly of sodium ferrous sulfate particles.

[0073] Example 1

[0074] The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided in an embodiment of the present invention comprises the following steps:

[0075] Step 1: Add 5% of the mass of sodium ferrous sulfate to Ketjen black in deionized water, the mass ratio of deionized water to Ketjen black is 5:1, and ultrasonicate for 12 hours to obtain a uniformly dispersed conductive carbon solution; ascorbic acid accounting for 20% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3 are respectively added to deionized water, the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, the mass ratio of the added deionized water to the sodium ferrous sulfate precursor is 5:1, and the mixture is fully stirred on a magnetic stirrer for 1 hour to obtain a light blue solution, and then the uniformly dispersed conductive carbon solution is poured into the light blue solution and mixed and stirred for 30 minutes to obtain a precursor solution;

[0076] Step 2: spray drying the precursor solution obtained in step 1 at 200° C. to obtain an intermediate phase;

[0077] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate composite positive electrode material.

[0078] The solid spherical sodium ferrous sulfate composite positive electrode material prepared in this embodiment is used to prepare the positive electrode plate. 1M NaClO4 is dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC) as the electrolyte. The outer shell is made of stainless steel, the negative electrode is made of metallic sodium sheet, and the positive electrode and the negative electrode are separated by a glass fiber separator to be assembled into a button battery.

[0079] Figure 5 This is the X-ray diffraction (XRD) pattern of the solid spherical sodium ferrous sulfate composite cathode material prepared in Example 1. It can match the standard X-ray diffraction standard card of sodium ferrous sulfate, that is, the Na 2.5 Fe 1.75 (SO4)3, indicating that the in-situ composite Ketjen black will not affect the sodium ferrous sulfate (Na 2.5 Fe 1.75 Synthesis of (SO4)3).

[0080] like Figure 6 As shown, a button cell assembled with a sodium ferrous sulfate cathode material containing 5% Ketjen black exhibits an initial discharge capacity of approximately 89 mAh / g at a C / 5 rate within a voltage range of 2.0 to 4.5 V. Compared to Comparative Example 1, the material's discharge capacity increased by approximately 21 mAh / g, and its cycling stability was significantly improved. This is because the added Ketjen black provides a highly efficient conductive carbon network, effectively enhancing the material's electronic conductivity.

[0081] Figure 7 This is a scanning electron microscope (SEM) image of a sodium ferrous sulfate cathode material composited with Ketjen Black. It shows that the Ketjen Black and sodium ferrous sulfate cathode materials are evenly mixed and self-assembled to form a solid spherical sodium ferrous sulfate composite cathode material with a dense structure and smooth surface.

[0082] Example 2

[0083] The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided in an embodiment of the present invention comprises the following steps:

[0084] Step 1: Add 5% of the mass of carbon nanotubes to deionized water, the mass ratio of the deionized water to the carbon nanotubes is 5:1, and ultrasonicate for 12 hours to obtain a uniformly dispersed conductive carbon solution; add ascorbic acid accounting for 20% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3 to deionized water, respectively, the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, the mass ratio of the added deionized water to the sodium ferrous sulfate precursor is 5:1, and stir thoroughly on a magnetic stirrer for 1 hour to obtain a light blue solution, and then pour the uniformly dispersed conductive carbon solution into the light blue solution and mix and stir for 30 minutes to obtain a precursor solution;

[0085] Step 2: spray drying the precursor solution obtained in step 1 at 200° C. to obtain an intermediate phase;

[0086] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate composite positive electrode material.

[0087] The solid spherical sodium ferrous sulfate composite positive electrode material prepared in this embodiment is used to prepare the positive electrode plate. 1M NaClO4 is dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC) as the electrolyte. The outer shell is made of stainless steel, the negative electrode is made of metallic sodium sheet, and the positive electrode and the negative electrode are separated by a glass fiber separator to be assembled into a button battery.

[0088] Figure 8 This is the X-ray diffraction (XRD) pattern of the solid spherical sodium ferrous sulfate composite cathode material prepared in Example 2. It can match the standard X-ray diffraction standard card of sodium ferrous sulfate, that is, the Na 2.5 Fe 1.75 (SO4)3, which shows that in situ composite carbon nanotubes will not affect the sodium ferrous sulfate (Na 2.5 Fe 1.75 Synthesis of (SO4)3).

[0089] like Figure 9 As shown, a button cell assembled with a sodium ferrous sulfate cathode material containing 5% carbon nanotubes exhibits an initial discharge capacity of approximately 80 mAh / g at a C / 5 rate within a voltage range of 2.0 to 4.5 V. Compared to Comparative Example 1, the material's discharge capacity increased by approximately 12 mAh / g, and its cycling stability was significantly improved. This is because the added carbon nanotubes provide a highly efficient conductive carbon network, effectively enhancing the material's electronic conductivity.

[0090] Figure 10 This is a scanning electron microscope (SEM) image of a sodium ferrous sulfate cathode material composited with carbon nanotubes. It can be seen that the carbon nanotubes and the sodium ferrous sulfate cathode material are evenly mixed and self-assembled into solid spherical sodium ferrous sulfate composite cathode materials with a dense structure and smooth surface. However, it can be seen that the carbon nanotubes are difficult to mix evenly with the sodium ferrous sulfate, and the carbon nanotubes are partially aggregated.

[0091] Example 3

[0092] The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided in an embodiment of the present invention comprises the following steps:

[0093] Step 1: Add 5% of the mass of reduced graphene oxide to deionized water, the mass ratio of the deionized water to the reduced graphene oxide is 5:1, and ultrasonicate for 12 hours to obtain a uniformly dispersed conductive carbon solution; ascorbic acid accounting for 20% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3 are respectively added to deionized water, the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, the added mass ratio of the deionized water to the sodium ferrous sulfate precursor is 5:1, and the mixture is fully stirred on a magnetic stirrer for 1 hour to obtain a light blue solution, and then the uniformly dispersed conductive carbon solution is poured into the light blue solution and mixed and stirred for 30 minutes to obtain a precursor solution;

[0094] Step 2: spray drying the precursor solution obtained in step 1 at 200° C. to obtain an intermediate phase;

[0095] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate composite positive electrode material.

[0096] The solid spherical sodium ferrous sulfate composite positive electrode material prepared in this embodiment is used to prepare the positive electrode plate. 1M NaClO4 is dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC) as the electrolyte. The outer shell is made of stainless steel, the negative electrode is made of metallic sodium sheet, and the positive electrode and the negative electrode are separated by a glass fiber separator to be assembled into a button battery.

[0097] Figure 11 This is the X-ray diffraction (XRD) pattern of the solid spherical sodium ferrous sulfate composite cathode material prepared in Example 3. It can match the standard X-ray diffraction standard card of sodium ferrous sulfate, that is, the Na 2.5 Fe 1.75 (SO4)3, which shows that in situ composite reduction of graphene oxide will not affect the sodium ferrous sulfate (Na 2.5 Fe 1.75 Synthesis of (SO4)3).

[0098] like Figure 12 As shown, a button cell assembled with a sodium ferrous sulfate cathode material containing 5% reduced graphene oxide exhibits an initial discharge capacity of approximately 78 mAh / g at a C / 5 rate within a voltage range of 2.0 to 4.5 V. This represents an increase in the discharge capacity of the material by approximately 10 mAh / g compared to Comparative Example 1. This is because the added reduced graphene oxide provides an efficient conductive carbon network, improving the material's electronic conductivity.

[0099] Figure 13 This is a scanning electron microscope (SEM) image of a sodium ferrous sulfate cathode material composited with reduced graphene oxide. It can be seen that the reduced graphene oxide and the sodium ferrous sulfate cathode material form a solid spherical sodium ferrous sulfate composite cathode material with a rough surface. The reduced graphene oxide is locally over-aggregated, and the layered structure of the reduced graphene oxide makes it difficult to mix evenly with the sodium ferrous sulfate. This may be the reason for the limited capacity improvement of the sodium ferrous sulfate.

[0100] Comparative Example 2

[0101] The present invention discloses a method for preparing a solid spherical sodium ferrous sulfate positive electrode material by spray drying, comprising the following steps:

[0102] Step 1: Ascorbic acid accounting for 30% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3.5 are added to deionized water, wherein the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, and the mass ratio of the added deionized water to the sodium ferrous sulfate precursor is 4:1. The mixture is stirred on a magnetic stirrer for 1 hour to obtain a light blue solution, thereby obtaining a precursor solution;

[0103] Step 2: spray drying the precursor solution obtained in step 1 at 220° C. to obtain an intermediate phase;

[0104] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate positive electrode material.

[0105] This comparative example prepares a solid spherical sodium ferrous sulfate positive electrode material that is not composited with a conductive carbon material. The resulting sodium ferrous sulfate positive electrode material is used to prepare a positive electrode sheet. The positive electrode sheet is made by vacuum drying a slurry coated on aluminum foil at 120°C and then cutting it into pieces. The slurry is made by mixing the positive electrode material with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1, and then adding 1-methyl-2-pyrrolidone as a dispersant. The electrolyte is 1M NaClO4 dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC). The outer shell is a stainless steel shell, the negative electrode is a metal sodium sheet, and a glass fiber separator is used to separate the positive and negative electrodes. The battery is assembled into a button cell.

[0106] Figure 14 This is a scanning electron microscope (SEM) image of the sodium ion battery prepared with the sodium ferrous sulfate positive electrode material prepared in Comparative Example 2 without adding conductive carbon material after cycling at a C / 5 rate. It can be seen that the solid spheres are slightly broken after cycling, and it is proved that the prepared material has a solid spherical structure.

[0107] Example 4

[0108] The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided in an embodiment of the present invention comprises the following steps: Step 1: adding 7% of the mass of sodium ferrous sulfate to Ketjen black in deionized water, the mass ratio of the deionized water to the Ketjen black being 4:1, ultrasonicating for 12 hours to obtain a uniformly dispersed conductive carbon solution; ascorbic acid accounting for 30% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3.5 are respectively added to deionized water, the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, the mass ratio of the added deionized water to the sodium ferrous sulfate precursor being 4:1, and stirring on a magnetic stirrer for 1 hour to obtain a light blue solution, and then pouring the uniformly dispersed conductive carbon solution into the light blue solution and mixing and stirring for 30 minutes to obtain a precursor solution;

[0109] Step 2: spray drying the precursor solution obtained in step 1 at 220° C. to obtain an intermediate phase;

[0110] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate composite positive electrode material.

[0111] The solid spherical sodium ferrous sulfate composite positive electrode material prepared in this embodiment is used to prepare the positive electrode plate. 1M NaClO4 is dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC) as the electrolyte. The outer shell is made of stainless steel, the negative electrode is made of metallic sodium sheet, and the positive electrode and the negative electrode are separated by a glass fiber separator to be assembled into a button battery.

[0112] like Figure 15 The figure shows a transmission electron microscope (TEM) image of the sodium ferrous sulfate composite cathode material, which shows that the synthesized sodium ferrous sulfate composite material is a solid spherical structure.

[0113] Example 5

[0114] The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided in an embodiment of the present invention comprises the following steps:

[0115] Step 1: Add 7% of the mass of carbon nanotubes to the mass of sodium ferrous sulfate into deionized water, the mass ratio of the deionized water to the mass of the carbon nanotubes is 4:1, and ultrasonicate for 12 hours to obtain a uniformly dispersed conductive carbon solution; ascorbic acid accounting for 30% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3.5 are respectively added to deionized water, the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, the mass ratio of the added deionized water to the mass of the sodium ferrous sulfate precursor is 4:1, and the mixture is fully stirred on a magnetic stirrer for 2 hours to obtain a light blue solution, and then the uniformly dispersed conductive carbon solution is poured into the light blue solution and mixed and stirred for 30 minutes to obtain a precursor solution;

[0116] Step 2: spray drying the precursor solution obtained in step 1 at 220° C. to obtain an intermediate phase;

[0117] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate composite positive electrode material.

[0118] The solid spherical sodium ferrous sulfate composite positive electrode material prepared in this embodiment is used to prepare the positive electrode plate. 1M NaClO4 is dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC) as the electrolyte. The outer shell is made of stainless steel, the negative electrode is made of metallic sodium sheet, and the positive electrode and the negative electrode are separated by a glass fiber separator to be assembled into a button battery.

[0119] Example 6

[0120] The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material by spray drying provided in an embodiment of the present invention comprises the following steps:

[0121] Step 1: Add 7% of the mass of reduced graphene oxide to sodium ferrous sulfate in deionized water, the mass ratio of the deionized water to the reduced graphene oxide is 4:1, and ultrasonicate for 12 hours to obtain a uniformly dispersed conductive carbon solution; ascorbic acid accounting for 30% of the mass of ferrous sulfate heptahydrate, anhydrous sodium sulfate and ferrous sulfate heptahydrate in a stoichiometric ratio of 2:3.5 are respectively added to deionized water, the ferrous sulfate heptahydrate is prepared by a wet process based on hematite sediment, the added mass ratio of the deionized water to the sodium ferrous sulfate precursor is 4:1, and the mixture is fully stirred on a magnetic stirrer for 2 hours to obtain a light blue solution, and then the uniformly dispersed conductive carbon solution is poured into the light blue solution and mixed and stirred for 30 minutes to obtain a precursor solution;

[0122] Step 2: spray drying the precursor solution obtained in step 1 at 220° C. to obtain an intermediate phase;

[0123] Step 3: Grind the intermediate phase evenly and then calcine it in a tube furnace under argon for 12 hours to obtain a spherical sodium ferrous sulfate composite positive electrode material.

[0124] The solid spherical sodium ferrous sulfate composite positive electrode material prepared in this embodiment is used to prepare the positive electrode plate. 1M NaClO4 is dissolved in ethylene carbonate (EC) / propylene carbonate (PC) / fluoroethylene carbonate (FEC) as the electrolyte. The outer shell is made of stainless steel, the negative electrode is made of metallic sodium sheet, and the positive electrode and the negative electrode are separated by a glass fiber separator to be assembled into a button battery.

[0125] Although the present invention has been described above with reference to the exemplary embodiments and the accompanying drawings, it will be apparent to those skilled in the art that various modifications may be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material, characterized in that: Step 1: Add the conductive carbon material to deionized water and sonicate for 6-12 hours to obtain a uniformly dispersed conductive carbon solution; add ascorbic acid, anhydrous sodium sulfate, and ferrous sulfate heptahydrate prepared based on hematite sediment to the deionized water respectively, and stir thoroughly on a magnetic stirrer to obtain a light blue solution; Then, the conductive carbon solution was poured into the light blue solution in a mass ratio of 1:1 and stirred to obtain a precursor solution; Step 2: spray drying the precursor solution to obtain the intermediate phase Na2Fe(SO4)2·4H2O; Step 3: grinding the intermediate phase uniformly and placing it in a tube furnace, calcining it under inert atmosphere to obtain solid spherical sodium ferrous sulfate Na 2.5 Fe 1.75 (SO4)3 composite positive electrode material.

2. The method for preparing the solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: The size of the solid spherical sodium ferrous sulfate composite positive electrode material particles is 0.5-10 microns.

3. The method for preparing the solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: The ferrous sulfate heptahydrate in step 1 is prepared by a wet production process based on hematite sediment, and the wet production process includes leaching, reduction and crystallization.

4. The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: In step 1, the rotation speed of the magnetic stirrer is 300-1000 rpm, and the stirring time is 1-10 h.

5. The method for preparing the solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: The conductive carbon material is at least one of Ketjen black, carbon nanotubes, graphene, acetylene black, graphite, and SuperP (conductive carbon black).

6. The method for preparing the solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: The mass of the conductive carbon material added is 3%-10% of the mass of the obtained sodium ferrous sulfate composite positive electrode material.

7. The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: The stoichiometric ratio of the anhydrous sodium sulfate to the ferrous sulfate heptahydrate is 2:(3-4); the added mass of the ascorbic acid is 20%-60% of the mass of the ferrous sulfate heptahydrate.

8. The method for preparing a solid spherical sodium ferrous sulfate composite positive electrode material according to claim 1, characterized in that: The spray drying temperature is set to 180-250° C.; the calcination temperature is 350-400° C.; the calcination time is 8-18 hours; and the inert atmosphere is argon or nitrogen or a mixture of an inert gas and a reducing gas.

9. A solid spherical sodium ferrous sulfate composite positive electrode material, characterized by: The solid spherical sodium ferrous sulfate composite positive electrode material is prepared by the preparation method of the solid spherical sodium ferrous sulfate composite positive electrode material according to any one of claims 1 to 8.

10. A sodium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode and the negative electrode are located on both sides of the separator, and the electrolyte is located between the positive electrode and the negative electrode, characterized in that: The positive electrode is a positive electrode sheet prepared by a solid spherical sodium ferrous sulfate composite positive electrode material prepared by the preparation method of a solid spherical sodium ferrous sulfate composite positive electrode material according to any one of claims 1 to 8, and the positive electrode sheet is a round sheet with a diameter of 14 mm; the negative electrode is a sodium metal sheet, wherein the diameter of the sodium metal sheet is 16 mm and the thickness is 0.2 to 0.4 mm; the electrolyte is 0.1 mL of a sodium perchlorate / ethylene carbonate / propylene carbonate / fluoroethylene carbonate solution with a concentration of 1 mol / L; and the diaphragm is a glass fiber diaphragm with a diameter of 19 mm.

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